[Autofill Assistant] MVC refactor of autofill assistant payment request.

This change aims to at least partially move PR to use the MVC pattern. There are still some properties which should be in native, but are not (in particular, the chosen payment/contact info/address and the set of available profiles & credit cards). I plan to address them in a follow-up CL.

This change is a refactorization - there should be no user-facing changes.

Bug: 806868
Change-Id: Iab88f7f35ba746194344ab11e367401f1daea61a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604045
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659969}
20 files changed